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Monday, April 4, 2011

Best laid plans...

Anyone who has flown Southwest Airlines -- or flew Piedmont Airlines in the 1980s -- is very familiar with the Boeing 737, the best-selling airliner in history. The original 737s entered service in the 1960s. Up-to-date models of the 737 are still being manufactured. If I had a dollar for every hour I've sat in a 737, I could afford a Château Margaux 1961.

Because most models of the 737 are optimized for short to medium range, they tend to have lots of takeoffs and landings. Aerospace engineers refers to those takeoffs and landings as cycles. It's not uncommon for 737s to fly 50,000 cycles or more before they are recycled into spare parts and soda cans.

Why do cycles matter? Because the interiors of passenger aircraft are pressurized, so that passengers can breathe while the aircraft flies efficiently at high altitudes (the proverbial 33,000 feet). The air at 33,000 feet is too thin to sustain your life. But if you were to bring on board an altimeter and check it from your passenger seat, it would read no higher than 8,000 feet. At that height, you can still breathe.

What's the trick? The aircraft pumps enough air into the cabin -- in a sense, over-filling the cabin with air, relative to what's outside -- so that there is adequate air for your lungs, regardless of how thin the air is outside.

In the process, however, the aircraft is subjected to stress. There's a lot of pressure differential between air at 8,000 feet and air at 33,000 feet. Unlike a balloon being inflated, the aircraft made of metal does not expand as it's filled with air. Rather, the aircraft undergoes tension, an engineering term for being stretched. When the aircraft descends prior to landing, the tension on the airframe is released. Over a long period of time, these cycles of tension can weaken the airframe -- especially where there are scratches or imperfections.

That's what caused the decompression of a Southwest 737 several days ago, and it's why the FAA is concerned about all 737s that have more than a certain number of cycles. Thus the FAA asked today that all such aircraft be inspected closely for signs of deterioration that could lead to decompressions if not repaired.

For many years Southwest has operated only 737s. It's a strategy essential to their financial success. Their pilots must learn how to fly only one aircraft, their mechanics must learn how to repair only one aircraft, and their logistics system must stock parts for only one aircraft. That's quite different from most airlines which operate many aircraft types. But there is a risk in putting all one's eggs in one basket, and that's what Southwest is experiencing now. If you're booked on a Southwest flight this week, good luck. And by the way, other airlines won't take Southwest tickets.